1. Field of the Invention
The present invention relates to a crystal oscillation device and an electronic device, and more particularly, to a crystal oscillation device, for example, used as a reference signal source in a PLL (Phase-Locked Loop) circuit included in a RF (Radio Frequency) circuit of a portable telephone and to an electronic device including the crystal oscillation device.
2. Description of the Related Art
In a conventional crystal oscillation device used as a reference signal source, a crystal oscillator is used as a resonator, and circuit components that define an oscillation circuit, a temperature-compensating circuit, and other suitable components are combined with the crystal oscillator. The conventional crystal oscillator normally has a structure in which a crystal strip with an electrode is hermetically mounted in a package made of ceramic or other suitable material. The circuit components need to be mounted on a circuit board made of ceramic or other suitable material. By combining the crystal oscillator and the circuit board by any means, an oscillation device is constructed.
Crystal oscillation devices in which a crystal oscillator and a circuit board are combined are taught in Japanese Unexamined Patent Application Publication Nos. 2001-177345, 2002-84138, and 2002-64333.
In a crystal oscillation device disclosed in Japanese Unexamined Patent Application Publication No. 2001-177345, circuit components are mounted on wires provided at the bottom of a recess formed in a ceramic box defining a circuit board, and a crystal oscillator package having almost the same shape is placed on and combined with the ceramic box. That is, the crystal oscillator is supported by four side walls of the ceramic box.
In Japanese Unexamined Patent Application Publication No. 2002-84138, a crystal oscillator having external terminals protruding from the bottom surface to a certain height is mounted on a crystal-oscillator mounting electrode provided on a flat thin circuit board. There is a space between the circuit board and the crystal oscillator corresponding to the height of the external terminals, and a circuit component is mounted in the space. An integrated circuit is used as the circuit component in order to achieve a low profile. The space between the circuit board and the crystal oscillator is filled with underfill resin so as to cover the circuit component and the external terminals of the crystal oscillator. While external terminals are also provided on the bottom surface of the circuit board, they do not overlap the external terminals of the crystal oscillator in plan view.
In Japanese Unexamined Patent Application Publication No. 2002-64333, columnar members are mounted on a flat thin circuit board, and a crystal oscillator is disposed on the columnar members. The columnar members also electrically connect the circuit board and the crystal oscillator and may be provided integrally with or separate from a package of the crystal oscillator. The positions of the columnar members coincide with the positions of external terminals provided on the bottom surface of the circuit board in plan view.
In Japanese Unexamined Patent Application Publication No. 2001-177345, because the circuit board is formed of a ceramic box, it is difficult to reduce the cost thereof. Moreover, it is much more difficult to mount circuit components on the bottom of the recess of the box-shaped circuit board than to mount the components on a flat board, and it is also difficult to reduce the operation cost. In addition, the area of the circuit board must be increased corresponding to the thicknesses of the four side walls of the box-shaped circuit board, and this hinders size reduction of the crystal oscillation device.
In Japanese Unexamined Patent Application Publication No. 2002-84138, because the external terminals protrude from the case of the crystal oscillator, the cost of the crystal oscillator is increased.
In Japanese Unexamined Patent Application Publication No. 2002-64333, because the crystal oscillator is supported only by the columnar members, the strength is insufficient. While the strength can be increased by forming the columnar members integrally with the package of the crystal oscillator, the cost of the package is increased.